The transcription factor OsbHLH55 regulates leaf inclination through the OsMAPK6–OsWRKY53 signaling pathway in rice

Leaf inclination, an essential characteristic of ideal plant architecture, plays a crucial role in determining crop yield by influencing leaf area index and planting density. Brassinosteroid (BR) serves as the major phytohormone involved in regulation of leaf inclination, while the mechanism of BR signaling regulation in rice still needs exploration when compared with Arabidopsis. Here, a basic helix-loop-helix (bHLH) transcription factor OsbHLH55 was found to be highly expressed in the lamina joint. osbhlh55 gene-edited mutants exhibited large leaf inclination resulting from increased size of parenchyma cells on the adaxial side of the lamina joint, while OsbHLH55 overexpression lines showed small leaf inclination. In addition, the osbhlh55 mutants were hypersensitive to BR, and expression of OsbHLH55 was inhibited by BR, suggesting that OsbHLH55 was negatively regulated by BR. Genetic experiments verify that OsbHLH55 can directly bind to the promoter of OsWRKY53 to repress its expression, and OsWRKY53 is involved in the BR signaling pathway. Moreover, OsbHLH55 can interact with and be phosphorylated by OsMAPK6 in vitro. Collectively, our investigation revealed OsbHLH55 as a negative regulator of rice BR signaling functioning in leaf inclination regulation, and sheds light on the intricate interplay between BR and the mitogen-activated protein kinase (MAPK) signaling pathway in rice.